The present invention relates to a biaxially stretched polyester film, and more particularly, to a biaxially stretched polyester film which can be suitably used as a packaging material for industrial materials, drugs, sanitary materials, foods, etc., and can exhibit not only a less deviation in thickness but also a good hand cut-off property.
Conventional packaging materials for industrial materials, drugs, sanitary materials, foods, etc., have been frequently required to have a good hand cut-off property. For example, bag-shaped packaging materials for confectioneries, powdery drugs, etc., having a good hand cut-off property are largely advantageous because of facilitated removal of contents therefrom.
As the material having such a good hand cut-off property, there are known cellophane, a so-called moisture-proof cellophane obtained by coating cellophane with a vinyl chloride-vinyl acetate copolymer, a film obtained by coating cellophane with vinylidene chloride (K-coat cellophane), etc.
The cellophane, moisture-proof cellophane, K-coat cellophane, etc., are excellent in hand cut-off property, but tend to suffer from change in their properties as well as deteriorated printability depending upon ambient humidity. In addition, the cellophane as a base material is expensive, and it is doubtful whether or not stable supply of the cellophane can be ensured in future. Further, the K-coat cellophane has a possibility of generating dioxins upon burning and, therefore, tends to be difficult to use from the viewpoint of avoiding environmental pollution. On the other hand, the use of aluminum foils also tends to be restrained from the viewpoint of avoiding environmental pollution.
With such a recent tendency, there has been proposed the packaging material having a good hand cut-off property which is made of a polyester film in place of cellophane (refer to Japanese Patent Application Laid-open (KOKAI) No. 5-104618). The polyester film has been frequently used as a packaging material because of excellent properties thereof such as mechanical properties, dimensional stability, heat resistance, water resistance, transparency, etc., but has a poor hand cut-off property owing to the excellent mechanical properties.
To solve problems due to the poor hand cut-off property of the polyester film, there have been proposed, for example, the monoaxially oriented polyester film (refer to Japanese Patent Publication (KOKOKU) No. 55-8551), the film made of a polyester resin obtained by copolymerizing polyester with diethylene glycol, etc., (refer to Japanese Patent Publication (KOKOKU) No. 56-50692), and the polyester film produced from a low-molecular weight polyester resin (refer to Japanese Patent Publication (KOKOKU) No. 55-20514). However, the monoaxially oriented polyester film is readily linearly cut off in the oriented direction, but tends to be hardly cut off in the other directions. The polyester film made of a polyester resin obtained by copolymerizing polyester with diethylene glycol, etc., has such a problem that inherent properties of the polyester are lost by the copolymerization. Further, the polyester film produced from a low-molecular weight polyester resin tends to suffer from troubles in its production process such as cutting or breakage of the film upon a stretching step, and, therefore, tends to be unpractical.
In addition, there has been proposed the method of allowing an orientation breaking layer to intervene in a polyester film (refer to Japanese Patent Application Laid-open (KOKAI) No. 5-104618 and Japanese Patent Application No. 2002-371183). However, in this method, since it is difficult to uniformly control the orientation breaking rate, there tends to arise such a problem that the thickness of the film is largely deflected as compared to a biaxially stretched polyester film (PET film) made of polyethylene terephthalate solely.
Further, there have been proposed the method of mixing an amorphous polyester in a polyester film (Japanese Patent Application Laid-open (KOKAI) No. 2003-155403), and the method of allowing an amorphous polyester layer to intervene in a polyester film (refer to Japanese Patent Application Laid-open (KOKAI) No. 2003-220678). However, when formed into a film, the amorphous polyester tends to suffer from larger deflection in thickness as compared to a crystalline polyester. Therefore, in these methods, it may also be difficult to obtain films exhibiting a less deflection in thickness similar to PET films.
In addition, there has been proposed the method of mixing polybutylene terephthalate (PBT) having a high crystallinity in a polyester film (refer to Japanese Patent Application No. 2002-300428). However, even though polybutylene terephthalate is simply mixed with other polyesters as proposed by this method, it may be difficult to improve the deflection in thickness of the film to an extent similar to that of PET films.